1. Field of the Invention
This invention relates to a multi-drop communications link including a master and a plurality of slave devices in which potential fault or spark conditions are avoided to render the communications link intrinsically safe for use in hazardous locations such as at the surface of an oil or gas well.
2. Description of the Prior Art
Multi-drop communication links between a master and a pluarlity of slave devices as depicted in FIG. 1. For example, a master device such as a mini computer can be interconnected with a plurality of non-intelligent or slave devices, such as printers or other display output, by transmit and receive lines. Such conventional multi-drop communication links are not, however, suitable for use in a hazardous location requiring intrinsically safe circuit. An intrinsically safe circuit has been defined by Underwriters Laboratory, Inc. in UL Standard 913 as a circuit in which any spark or thermal effect, produced normally or in specified fault conditions, is incapable in the test conditions prescribed in this Standard of causing ignition of a mixture of flammable or combustible material in the air in its most easily ignited concentration. That same Standard defines an intrinsically safe apparatus as an apparatus in which any spark or thermal effect produced either normally or in specified fault conditions, is incapable, under test conditions prescribed in this Standard, of causing ignition of a mixture of flammable or combustible material in air in its most easily ignitable concentration. The energy inflamable in a hazardous location must not be capable of igniting a hazardous atmosphere mixture due to arcing or temperature during normal operations. Normal operations includes opening, shorting and grounding of field wiring of an intrinsically safe circuit. A fault is defined as a defect or electrical breakdown of any component, spacing, or insulation either alone or in combination with other faults may adversily affect the electrical or thermal characteristics of the intrinsically safe circuit. If a defect or breakdown leads to defects or breakdowns in other components, the primary and subsequent defects and breakdowns are considered to be a single fault.
The most common method of insuring intrinsic safety is through the use of a fuse-protected shunt-diode safety barrier, which is defined as a network consisting of a series fuse, voltage-limiting shunt-diodes, and a current-limiting resistor designed to limit current and voltage. The fuse protects the diodes from open-circuiting in a high fault current condition.
Reliance upon safety barriers alone to provide an intrinsically safe multi-drop link would require the use of safety barriers as shown in FIG. 2. Such a barrier multi-drop link would require the use of a barrier on each transmit line leading from each component in the multi-drop loop and a barrier on each receive line leading from each component in the multi-drop loop. Thus, two barriers would be required for each component, master and slave, for a multi-drop loop. This barrier multi-drop approach is, however, severely limited in a current loop application because the potential across each barrier associated with the transmit side of the master, and thus the power required to the slaves, is additive. Therefore, the number of safety barriers which can be used in a hazardous enviroment is limited. Therefore, the use of conventional safety barriers alone limits the number of components which can be used in a hazardous location if intrinsic safety is to be maintained.
The instant invention relies not only upon conventional safety barriers, but provides adequate clearance distances for components within the communications link to define an intrinsically safe circuit for use in a hazardous environment. This is especially important in providing a multi-drop communication link which can be used at the surface of a subterranean oil or gas well. For example, operators of offshore drilling rigs require that intrinsic safety be maintained on these offshore rigs since any potential blowout or fire can result in significant loss. Any electrical system used to monitor drilling activity and the condition of the well bore must therefore be intrinsically safe.